Ammonium nitrate particulate fertilizer and method for producing the same

ABSTRACT

An improved particulate, ammonium nitrate is prepared with 2.0-6.0 percent by weight potassium sulfate. The potassium sulfate suppresses crystalline phase IV⃡III transitions in the ammonium nitrate particles while undergoing temperature variations between 18°-42° C. The resulting fertilizer, which may be further coated to further suppress dusting, exhibits superior resistance to caking, clumping and cracking.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to particulate ammonium nitrate fertilizer. Morespecifically, ammonium nitrate fertilizer is rendered more resistant tobreakdown into fines, to particle bridging, and to caking by theaddition of potassium sulfate. The resulting fertilizer does not exhibitthe IV⃡III crystalline phase transition at ambient storage conditions asdoes ammonium nitrate without the potassium sulfate additive.

2. Background of the Prior Art

Particulate ammonium nitrate is a commonly used plant fertilizer.Usually provided in nearly spherical particles referred to as prills,this fertilizer typically provides 34% nitrogen by weight to the cropsbeing fertilized. Under cool, dry storage conditions, the productremains in good physical condition. However, under summer storageconditions of 25°-42° C. and 40-90 percent relative humidity,particulate ammonium nitrate has been shown to suffer physicaldegradation, including particle breakdown, bridging, and cracking.

High-Density (Hi-D) ammonium nitrate particles, or prills, are producedfrom melt containing 0.3% water, or less; therefore, the prills areessentially anhydrous. Ammonium nitrate is, however, hygroscopic andwill absorb moisture during storage. Fertilizer particles in the absenceof "free" (i.e. unbound) water do not undergo the IV⃡III crystal phasetransitions. As sufficient moisture is absorbed from the ambient air,the IV⃡III phase change can occur; the precise temperature of the changevaries with the amount of water present. (Griffith, J., Chem. Eng. Data,8:22 (1963).) Additives, used by some ammonium nitrate producers, whichbind moisture in the form of a hydrate (e.g.; Mg(NO₃)₂ +2H₂ O→Mg(NO₃)₂2H₂ O), prevent the absorbed moisture from remaining in a free state.Therefore, absorbed moisture must exceed the amount necessary to fullyhydrate the additive before it can initiate the IV⃡III phase transition.

There is a substantial particle volume change associated with the IV⃡IIIcrystal phase change (Miller, P. et al., Industrial and EngineeringChemistry, Vol. 40, No. 1, pp. 154-160 (1948)). As the storagetemperature cycles after free water has become present, the particlesalternatively swell and shrink, inducing formation of cracks in theparticles themselves, and crystals from separate particles, fusing intoa single mass. Repeated phase IV⃡III changes are also associated withproduction of a large amount of undesirable "fines".

Various approaches have been adopted to address particulate ammoniumnitrate fertilizer's physical instability. In addition to the use ofhydrating agents such as MgNO₃ and (Al)₂ (SO₄)₃, a great variety ofother additives have been employed, including nitrates, phosphates,sulfates and oxides of various metals. (See, e.g., Russo, ProductResearch and Development 7:69 (1968) as well as U.S. Pat. Nos. 3,630,712and 3,018,164.) An alternative approach is described in U.S. Pat. No.4,521,239, commonly assigned herewith, and incorporatedherein-by-reference. In this patent, the prills, formed from aconcentrated ammonium nitrate solution to which is added magnesiumnitrate, are coated with the reaction product of an amine and asiloxane. The coating is effective in suppressing moisture absorption bythe ammonium nitrate. However, it has been impractical thus far to applyenough coating to render the prills completely impervious to moistureabsorption.

French patent publication 2,061,631 (and the equivalent teaching inGerman Offenlegensschrift 3,044,752) describe the suppression of crystalphase IV⃡III changes in particulate ammonium nitrate through the additionof 10 percent-50 percent, (wt/wt) potassium sulfate (K₂ SO₄). Thepublication specifically requires a minimum of 10 percent potassiumsulfate, (see page 4 of the references) indicating that below thisminimum, no appreciable suppression of the phase change is achieved. Thereference notes that ammonium nitrate particles not containing at least10% potassium sulfate, can experience a 3.6 percent volume change duringthe IV⃡III phase transition at 32° C.

The addition of 10-50 percent of potassium sulfate (wt/wt) to commercialammonium nitrate fertilizers is not economically feasible. Too muchnitrogen is replaced in the product, rendering it less competitiveagainst fertilizers having a higher nitrogen content. Preferably, nomore than 6 percent by weight, of potassium sulfate, and morepreferably, less than 4 percent by weight, of ammonium nitrate should bereplaced by additives intended to stabilize the ammonium nitrateparticles and yet remain economically competitive.

Accordingly, it remains an object of those of skill in the art toprovide a more highly stabilized particulate ammonium nitratefertilizer, which resists IV⃡III phase changes at the typical 25°-42° C.,which exhibits reduced cracking, caking, and production of increasedfines during storage.

SUMMARY OF THE INVENTION

The above objects, and others made more evident in light of the detaileddisclosure set forth below, is met by providing an ammonium nitrateparticulate fertilizer which contains between 2.0-6.0 weight percentpotassium sulfate. The addition of 2.0-6.0 percent potassium sulfate tothe ammonium nitrate dramatically suppresses IV⃡III crystalline phasetransitions of the ammonium nitrate during temperature cycles in theambient storage. The particulate ammonium nitrate is made according tothe general process described in U.S. Pat. No. 4,521,239. An ammoniumnitrate solution is prepared by reacting ammonia with nitric acid. Theammonium nitrate solution is then concentrated to--in general--above 99%ammonium nitrate. In some conventional processes, this concentratedsolution is reacted with magnesium oxide to produce a mixture ofammonium nitrate and magnesium nitrate. In the inventive process,magnesium may be added, but is not required. 2.0-6.0 weight percentpotassium sulfate is added to the melt and then formed into particles,by prilling, prill "fattening", granulation, etc.

The potassium sulfate may be in the form of a dry salt (K₂ SO₄), amixture of the salt, or in separate components; e.g., potassium nitrateand sulfuric acid.

The ammonium nitrate fertilizer of the claimed invention has improvedresistance to caking, swelling, cracking and other fines-forming eventscaused by the subsequent IV⃡III transitions during storage.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 depicts differential scanning calorimeter (DSC) scans from 24°C.-200° C. of lab-prilled pure ammonium nitrate (used as a testingcontrol sample). FIG. 1A shows results where prill moisture equals 0.57%and FIG. 1B shows results where moisture equals 1.64%.

FIG. 2 depicts differential scanning calorimeter (DSC) scans oflab-prilled ammonium nitrate containing 2.4% reagent-grade ammoniumsulfate as an additive. FIG. 2A shows results where prill moistureequals 0.23% and FIG. 2B shows results where moisture equals 1.51%.

FIG. 3 depicts differential scanning calorimeter (DSC) scans oflab-prilled ammonium nitrate containing 3.2% technical grade potassiumnitrate. FIG. 3A shows results where moisture equals 0.23% and FIG. 3Bshows results where moisture equals 1.94%.

FIG. 4 depicts a differential scanning calorimeter (DSC) scan oflab-prilled ammonium nitrate containing 2.3% technical grade potassiumsulfate. FIG. 4A shows the results where moisture equals 0.26%, and FIG.4B shows the results where moisture equals 2.30%.

FIG. 5 depicts results obtained from the Moisture Absorption test,demonstrating the rates at which the particles will absorb moisture fromthe atmosphere.

FIGS. 6 and 7 depict results from the Burrell Shaker test. This testmeasures the resistance of the ammonium nitrate particles to breakagecaused by a combination of repeated thermal and physical shocks.

DETAILED DESCRIPTION OF THE INVENTION

The improved particulate ammonium nitrate fertilizer of this inventionis characterized by the presence of 2.0-6.0 weight percent potassiumsulfate. As made clear in the examples set forth below, the potassiumsulfate suppresses the tendency of the ammonium nitrate to undergo IV⃡IIIcrystalline phase transitions, which otherwise occur at ambient storageconditions of 25°-42° C. and relative humidity values of 40-90%. Below2.0% potassium sulfate, suppression of the IV⃡III phase transition is notcompletely achieved; however, it is suppressed to some degree. Thus,ammonium nitrate fertilizers with less than about 2.0% potassium sulfatewill continue to exhibit a tendency to generate fines, to clump, cake,and to crack. No further improvement in the suppression of the phasetransition is observed above 6.0% potassium sulfate. Also, thefertilizer begins to suffer from loss of nitrogen content, due to thedilution effect of potassium sulfate. While potassium sulfate has beenused in the prior art practices as a source of sulfur for fertilizers,such does not embrace the present invention since the amount ofpotassium and sulfur are very small in comparison to "fertilization"amounts.

Other conventional ammonium nitrate fertilizer additives may also beemployed, but do not constitute an inventive aspect of the invention,per se. While phase transition is suppressed across the range of 2.0-6.0weight percent potassium sulfate, 2.5-3.5 percent is a preferred range.This amount offers excellent suppression of phase transition, whilemaintaining the weight percentage of potassium sulfate sufficiently lowsuch that the percentage of nitrogen in the ammonium nitrate fertilizerremains high.

The suppression of the IV⃡III phase transition eliminates the 4% volumeincrease that occurs during storage. Without the volume increase, theammonium nitrate particles will not absorb significant amounts of fueloil or related oily compositions.

The process for preparing ammonium nitrate particulate fertilizer isessentially that disclosed in U.S. Pat. No. 4,521,239. The potassiumsulfate is advantageously added to the ammonium nitrate solution at alocation in the process following the reaction between nitric acid andammonia. For ease of practice, the potassium sulfate is preferably addedafter concentration of the ammonium nitrate solution, such thatconcentrations of the ammonium nitrate are above 90%, typically above95%, and most preferably at or above 99%. The potassium sulfate must beadded to the ammonium nitrate prior to its formation into coolparticulate. Magnesium oxide, aluminum oxide, and other additives cansimilarly be added with the potassium sulfate to the concentratedammonium nitrate solution, or melt. This provides for thorough, uniformdistribution of the materials throughout the prills, leading to a highand consistent quality.

The effectiveness of the invention in suppressing formation of fines,and in particularly reducing fertilizer caking, breaking down intofines, and cracking, is more fully illustrated in the examples set forthbelow.

EXAMPLE 1 Preparation of Test Particles

The bulk of the research for this invention was performed on ammoniumnitrate containing the internal additive, magnesium nitrate. Experimentswere also performed on ammonium nitrate (i.e., reagent grade) withoutthe internal additive; and similar results were obtained. The ammoniumnitrate melt containing magnesium nitrate for the research was obtainedfrom the production facility down-stream of the evaporator.Magnesium-free ammonium nitrate melt was acquired at a point in theprocess upstream of the evaporator. The ammonium nitrate melt which didnot contain magnesium was concentrated in the laboratory to about 99.6%ammonium nitrate, i.e., equivalent to melt following theproduction-scale evaporator.

Varying amounts of additives (reagent and technical grades) were addedto about 1200 g of molten ammonium nitrate in a 1,000 ml beaker. Thecontents were stirred until the mixture was homogeneous. The beaker waswrapped with heating tape to provide a uniform temperature throughoutthe sample preparation. The solutions were sparged with anhydrousammonia to keep the pH between 6.4 and 6.9. The molten material was thentransferred to a stainless steel pan (24.1 cm×15.2 cm×5.1 cm) in whichan ammonia sparger was also present to maintain the pH of an 8% solutionof the melt in water at about 6.4. The temperature of the melt was keptat about 182° C. in the stainless steel pan. "Control" particles werealso made in the lab by the same procedure from ammonium nitrate meltobtained from the production facility.

Teflon sheets (20.32 cm×12.7 cm×0.3 cm) having 587 "dimples" were usedto prepare the test particles. The dimples (0.3 cm deep) were made usinga 0.278 cm common drill bit. Each dimple was pierced at the bottom witha #78 common drill bit. These holes facilitated removal of air bubblesfrom the dimples while they were being filled with melt. The dry Teflonsheets, initially at room temperature, were placed in the stainlesssteel pan containing the molten ammonium nitrate/additive. As thetemperature of the Teflon sheets increased, the molten salt flowed offthe top of the sheets leaving the dimples filled with the melt. TheTeflon sheets were then removed from the pan and placed in a desiccator.The laboratory-prepared particles were removed after the temperature ofthe sheets had decreased to room temperature.

The lab-prepared particles produced in the above-described manner wereuniformly sized, -6/+8 (U.S.A. Standard Testing Sieve), pellet-shapedproducts. Analytical data for the laboratory prepared products are givenin Table 1.

EXAMPLE 2 Differential Scanning Calorimeter (DSC)

The differential scanning calorimeter was used to determine productcrystalline form at a given temperature. When ammonium nitrate containspotassium sulfate at a concentration of at least 2.6 weight %, there isno IV⃡III crystalline phase transition observed in the DSC scan (FIG. 4).The DSC scans of (FIG. 1) ammonium nitrate (control), (FIG. 2) ammoniumnitrate/ammonium sulfate (at least 2.43 weight %), and (FIG. 3) ammoniumnitrate/potassium nitrate (3.18 & 3.4 weight %) all show four endotherms(Melt →I, I→II, II→III, III→IV). These results indicate that both thepotassium and sulfate ions are required to be present in ammoniumnitrate in order to suppress the IV⃡III crystalline phase transition ofammonium nitrate. When the particles are humidified, a concentration ofat least 2.5% potassium sulfate must be present to prevent the IV⃡IIIcrystalline phase occurrence. The only major difference observed in theDSC scans of the humidified products compared to the fresh product is anapproximate 18.8° C. decrease in the low temperature endotherm for thesample containing potassium nitrate (FIG. 3).

EXAMPLE 3 Moisture Absorption Test

The moisture absorption test requires that the ammonium nitrateparticles be exposed to a constant 81% relative humidity at labtemperature for a given time. The amount of moisture absorbed duringthat period is measured using a Karl Fischer moisture titrator. TheMoisture Absorption test demonstrates the relative rates at whichdifferent particle compositions will absorb moisture from theatmosphere; test results are given in FIG. 5. Slower moisture absorptionis preferred. Ammonium nitrate particles containing ammonium sulfate at1.43% having the lowest moisture absorption rate, followed by potassiumsulfate at 2.88%, and potassium nitrate at 3.18%. The "control" samplehad the highest moisture absorption rate.

EXAMPLE 4 Moisture Migration Test

The Moisture Migration Test determines the volume of expansion of 50 mlof bulk fertilizer particles as they are exposed to repeated temperaturecycling between the IV⃡III crystalline phases in an air-tight container.In theory, the Moisture Migration Test simulates the temperature cyclesthe particles would undergo in storage. The test consists of cycling theparticle temperature ten times between 25°→60°→25° C. These limitsencompass the temperature at which the IV⃡III crystalline phasetransition of ammonium nitrate occurs. Heating ammonium nitrate through25°-60° C., causes a 4% volume increase of the particles. The dataobtained from this test (FIG. 6) on pre-humidified particles show a 28%volume increase for the "control", a 36% increase for ammonium nitratecontaining ammonium sulfate, a 4% increase for ammonium nitratecontaining 3.18% technical-grade potassium nitrate, and no volumeincrease for ammonium nitrate samples containing 2.88% potassiumsulfate, or that containing 3.41% potassium nitrate. The absence of anincrease in volume for (NH₄)₂ NO₃ /reagent grade KNO₃ and the smallvolume change for (NH₄)₂ NO₃ /technical grade KNO₃ indicate that: (1)potassium ions alter the IV⃡III phase transition of ammonium nitrate; and(2) the concentration of potassium may affect the results. When the testwas completed, the products containing reagent grade potassium nitrate,potassium sulfate and technical grade potassium nitrate were slightlycaked, but retained excellent prill integrity. On the other hand, the"control" particles and particles containing ammonium sulfate wereseverely caked, with considerable prill degradation.

These results indicate that even humified particles containing as muchas 3.0% water will not experience a IV⃡III phase transition attemperatures that will be encountered in storage when ammonium nitratecontains 2.7% potassium sulfate.

EXAMPLE 5 Burrell Shaker Test

The Burrell Shaker test measures the resistance of ammonium nitratesamples to breakage caused by a combination of thermal and physicalshocks. In this test the samples are exposed to a temperature cyclebetween room temperature and 43° C., then mechanically shocked forthirty minutes via a Burrell Shaker. Prill degradation is then monitoredversus the number of cycles through which they are subjected. Thestandard Burrell Shaker test is considered complete after 300 cycles,even if the particles have not degraded appreciably. The ammoniumnitrate particles containing ammonium sulfate disintegrated much fasterthan the control. The only samples that did not totally break apart atless than 100 cycles are the ammonium nitrate/potassium nitrate andammonium nitrate/potassium sulfate samples. At the end of the test (300cycles), three of the four ammonium nitrate/potassium sulfate samplesremained completely intact and in excellent condition. One of theammonium nitrate/technical grade potassium sulfate samples started tochip after 264 cycles; however, at the conclusion of the tests only 8%of those particles were chipped. Four of the ammonium nitrate/technicalgrade potassium sulfate samples remained free-flowing throughout thetest.

The NH₄ NO₃ /technical grade KNO₃ and NH₄ NO₃ /reagent grade KNO₃samples started to chip after the 79th and 222nd cycles, respectively.By the end of the test, 46 and 20% of the particles containing reagentgrade potassium nitrate, and 18 and 12% of the technical grade potassiumnitrate particles had chipped. The particles in all four of thesesamples were stuck together each time the samples were checked. Thisclumping phenomenon probably helped reduce the number of particles thatbroke apart during the experiment. The testing was repeated with similarresults. The data from these experiments are shown in FIGS. 6 and 7.

Combining data from all tests revealed that potassium sulfate is thesuperior additive for increasing the time ammonium nitrate can remain inbulk storage. Product containing potassium sulfate (1) had the lowestmoisture absorption rate, (2) had the fewest particles broken duringBurrell Shaker, and (3) does not swell during the moisture migrationtest. When ammonium nitrate contains potassium sulfate at a level of 2.0weight percent, the quality of the product is similar to thosecontaining at least 2.6% potassium sulfate. However, the product willundergo a crystalline phase transition at storage temperatures. Theammonium nitrate product containing potassium nitrate also showedimprovement over ammonium nitrate. At the concentration investigated,the product will, however, experience a crystalline phase transition atstorage temperatures. The ammonium nitrate containing potassium nitrateparticles also stuck together during the Burrell Shaker test whichindicates serious caking of this product in storage.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

                                      TABLE 1                                     __________________________________________________________________________    Analytical Data for Lab-Prilled Ammonium Nitrate Containing Magnesium         Nitrate                                                                       with Secondary Additives as Shown                                                         Samples                                                                           (NH.sub.4).sub.2 SO.sub.4                                                             CaSO.sub.4                                                                          MgSO.sub.4                                                                           K.sub.2 SO.sub.4 -t                                                                 K.sub.2 SO.sub.4 -r                                                                  KNO.sub.3 -t                                                                        KNO.sub.3 -r          Analyses    Control                                                                           2.43    2.75  3.18.sup.1                                                                           2.88  2.82   3.18  3.41                  Additive, % 0.0 (1.77% SO.sub.4.sup.2)                                                                (1.94 SO.sub.4.sup.2)                                                               (2.5% SO.sub.4.sup.2)                                                                (1.295% k.sup.+)                                                                    (1.265% k.sup.+)                                                                     (1.23%                                                                              (1.32%                __________________________________________________________________________                                                            k.sup.+)              N, %        33.79                                                                             33.62.sup.2                                                                           33.14 33.14  33.02 31.09  32.93 32.88                 pH          6.7 6.0     6.2   6.7    6.5   6.9    6.7   6.6                   H.sub.2 O, % (Initial)                                                                    0.57                                                                              0.23    0.44  0.28    0.26  0.38  0.23  0.28                  Moisture Absorption                                                                       2.35                                                                              0.75    --    --     1.1   --     1.5   --                    (After exposure to 81%                                                        Relative at Room Temp.                                                        for 4 Hours                                                                   Moisture Migration                                                                        64  68      69    67.5   50    50     52    50                    initial volume 50 ml                                                          volume after 10 cycles                                                        Temperature of the                                                                        138.85                                                                            112.01  136.33                                                                              132.62 No    No     130.96                                                                              124.34                IV←→III crystalline phase                                                                              Transition                                                                          Transition                         transition, °F.                                                        __________________________________________________________________________     .sup.1 Corrected for Mg from MgO.                                             .sup.2 Adjusted for the amount of sulfuric acid generated from the            analysis. Amount of sulfur determined by the LECO SC430 analyzer              .sup.3 Prills taken from bulk samples that were humidified for the            moisture migration test. All samples were exposed to 81% humidity for 44      hours except K.sub.2 SO.sub.4 -t and (NH.sub.4).sub.2 SO.sub.4 which were     exposed for 72 hours.                                                    

What is claimed is:
 1. A fertilizer comprising prills of ammoniumnitrate, said prills consisting essentially of ammonium nitrate and fromabout 2.0-6.0 wt. % potassium sulfate.
 2. A fertilizer comprising prillsof ammonium nitrate, said prills consisting essentially of ammoniumnitrate, a magnesium compound as an internal additive and from about2.0-6.0 wt % potassium sulfate.
 3. The fertilizer of claim 2, whereinsaid prills exhibit substantially less caking, clumping anddisintegration into fines than ammonium nitrate prills without potassiumsulfate.
 4. The fertilizer of claim 2, wherein said potassium sulfate ispresent in about 2.5-3.5 weight percent.
 5. The fertilizer of claim 1,wherein said prills exhibit substantially no crystal phase IV⃡IIItransition when prills are exposed to hot, humid storage conditions. 6.The fertilizer of claim 2, wherein said hot, humid conditions range from18°-42° C. and 25-95 percent relative humidity.
 7. A method of makingthe fertilizer of claim 2, comprising:forming a concentrated solution ofammonium nitrate and magnesium nitrate as an internal additive, addingto said concentrated solution 2.0-6.0% by weight potassium sulfate, andcooling the resulting solution in a manner to provide solid particles ofammonium nitrate fertilizer.
 8. The process of claim 7, where saidprills are further coated with a reaction product of an amine and asiloxane, wherein said coating further stabilizes said fertilizerparticles against caking, cracking or clumping.
 9. The process of claim7, wherein said potassium sulfate is added in an amount of 2.5-3.5weight percent.
 10. A fertilizer comprising prills of ammonium nitrate,said prills consisting essentially of ammonium nitrate and from about2.0-6.0 wt. % potassium sulfate and coated with the reaction product ofan amine and a siloxane.
 11. A fertilizer comprising prills of ammoniumnitrate, said prills consisting essentially of ammonium nitrate, amagnesium compound as an internal additive and from about 2.0-6.0 wt. %potassium sulfate and coated with the reaction product of an amine andsiloxane.
 12. The fertilizer of claim 11, wherein said potassium sulfateis present in an amount of about 2.5-3.5 wt. %.
 13. The fertilizer ofclaim 11, wherein said prills exhibit substantially no crystal phaseIV⃡III transition when prills are exposed to hot, humid conditions. 14.The fertilizer of claim 13, wherein said hot, humid conditions rangefrom 18°-42° C. and 25-95% relative humidity.
 15. The fertilizer ofclaim 11, wherein said prills exhibit substantially less caking,clumping and disintegration into fines than ammonium nitrate prillslacking potassium sulfate.